1. Field of the Invention
The present invention relates to a transformer for mounting on a printed circuit board having high current and low current windings. Further, the invention relates to transformers which can be used on a printed circuit board to provide a gate drive voltage source for field effect transistors such as MOSFETs, or indeed other circuit signals.
2. Background Information
It has long been appreciated that stand alone transformers when mounted on printed circuit boards are very bulky and project too far above the circuit board so that various other techniques have been proposed to provide transformers and like devices that could be positioned more closely to the board and not project too far above the board. At the same time, it is essential that the transformer have good electrical characteristics and specifically low AC resistance and good magnetic coupling.
It is known to provide a planar transformer which includes a multilayer printed circuit board with a primary winding etched on one surface of the printed circuit board and a secondary winding etched on another surface. Such etching and forming is generally referred to simply as "printing" and thus is the term used in this specification. Such as planar transformer is described in British Patent Specification No. 2252208 (Burr-Brown Corporation). Unfortunately, such a transformer can have difficulties in relation to basic insulation. It is generally not suitable for automated assembly and its electrical performance is not as good as it should be. European Patent Specification No. 318,955 (John Fluke Manufacturing Co. Inc.) shows a similar power transformer.
British Patent Specification No. 2285892 (Advanced Power Conversion Limited) illustrates another such transformer in which the secondary winding is printed on the circuit board and the primary winding is provided on an auxiliary printed circuit board or substrate.
However, none of these have sufficiently good electrical properties, although the European Patent Specification No. 0 318 955 does refer to the need for better permeability so as to provide a flux path with the lowest reluctance. However, it is suggested that these constructions do not achieve the sane characteristics as more conventionally constructed transformers. Further, many of these power transformers are used on printed circuit boards which supply power for driving field effect transistors (FETs) for use as fast acting switches in synchronous rectifier applications. Essentially, synchronous rectifiers are an effective method for increasing converter efficiency and what is essential is to be able to condition an available signal into an appropriate form for driving the MOSFET at the appropriate voltage. The problem is that in many situations and power applications there is a transformer whose characteristics are determined by the remainder of the circuit and thus the use of the transformer as part of the driving circuit of such field effect transistors is to a certain extent a side issue. They are often not that efficient for that purpose.
A particular problem arises as is often the case when one of the windings has to handle relatively high currents. The power handling capabilities of printed windings is often inadequate.